1. Field of the Invention
The present invention relates to an optical modulating apparatus, and more particularly to an optical modulating apparatus having a bias controller and a bias control method, which can automatically perform a bias adjustment so as to keep the optical output characteristics of an optical modulator constant.
2. Description of the Related Art
In a dense wavelength division multiplexing (DWDM) optical transmission system, multiple optical signals having different wavelengths are transmitted through one optical fiber. Consequently, the DWDM optical transmission system achieves an improved transmission efficiency. Further, the DWDM optical transmission system can transmit optical signals regardless of their transmission speed. Therefore, the DWDM optical transmission systems are now employed in ultrahigh-speed Internet where the transmission amount has recently been increasing. In order to keep the transmission performance constant and reduce the differences in transmission performance among respective channels in such a DWDM optical transmission system, it is of the highest importance to keep the optical output characteristics of an optical modulator constant, in the transmitting end.
Generally, an optical modulator (using an electro-optic effect) has an on/off characteristic that varies according to an applied voltage. It also has a sine function characteristic and typically performs intensity modulation by a voltage Vπ for the on/off operation. The magnitude of an applied electric signal and the selection of an operation point are important, for a complete on/off operation of an optical signal magnitude-modulated through the optical modulator. This operation point is where the optical output characteristic that typically indicates a value of Vπ/2 becomes ½ of the original value, and where the intensity modulation is performed.
FIG. 1 schematically illustrates a general optical modulator. The optical modulator 10 has two interference type optical waveguides 10a and 10b provided therein. It shows a periodic optical output characteristic through a phase delay between the optical waveguides. This phase delay is induced by an RF electrode provided between the optical waveguides inside the optical modulator. Also, a separate electrode is provided in the optical modulator to adjust an operation voltage.
FIG. 2 illustrates the change of the optical output characteristic of the optical modulator of FIG. 1 according to a change of bias voltage. In FIG. 2, a required signal having a magnitude of Vπ for the on/off operation is applied. The operation point that is used to keep the constant output characteristic is indicated as a bold dotted line. This operation point acts as an important parameter that determines the transmission distance in accordance with a signal applying method.
Such an optical modulator may produce a minute change of the transfer characteristic in accordance with an external temperature and long-time operation. This change in the transfer characteristic directly causes a change in the operation point.
FIG. 3 illustrates the distortion of an output signal according to the change of the output characteristic of the optical modulator. In FIG. 3, if the transfer characteristic of the optical modulator (which performs the on/off modulation based on the operation point having a value of Vπ/2) is changed due to an external cause, the same output characteristic can not be obtained even though the operation point is kept the same. This implies distortion of the modulated signal. It is the role of a bias adjustment apparatus to locate the operation point to match the output characteristic, which changes due to an external change.
A bias adjustment method for a conventional optical modulator applies a pilot tone signal having a constant frequency unlike an information signal, e.g., an electric dithering signal, to an input optical signal.
FIG. 4 is a view explaining the concept of bias adjustment of a conventional optical modulator. In FIG. 4, if the operation point where the optical modulator is initially located is ‘A’, an average current 42 of a photo detector 41 located at the optical output terminal becomes zero when a complete on/off operation is performed. If the operation point is changed to ‘B’ or ‘C’ due to the change of the optical transfer characteristic according to the external change, a signal component 43 or 44 that is smaller or larger than the average current is produced. Accordingly, by detecting and tracking this signal component, the applied bias voltage is adjusted to the point where the average current becomes ‘0’, so that the optical output is kept constant.
However, the conventional optical modulator has drawbacks in that noise may be generated in the system. In particular, the pilot tone signal may have a relatively large amplitude in order to detect the distorted components of the signal, thus, the pilot tone signal may act as noise in regard to the information signal.